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and the other big one is there is nothing stopping the Yanks switching off the satellites.

I was going to say the same thing, but I understand that the European Galileo constellation is compatible with GPS and most modern nav systems should switch seamlessly to whichever system is available. Unlike GPS, Galileo is not a military system. But coverage in urban areas, and in tunnels etc. is always a problem. Some smartphones even use inertial navigation (INS) when their GPS is out of range (indoors).

I would still fit an analogue speedo, as Dirk suggests. You never know when the boys in blue might stop for a chat (it's happened to me).

This thread has put a spanner in the works for me. I was planning to fit a GPS speedo to my car. and hadn't given a second thought as to it's legitimacy, but now after reading these comments I'm having second thoughts.

This thread has put a spanner in the works for me. I was planning to fit a GPS speedo to my car. and hadn't given a second thought as to it's legitimacy, but now after reading these comments I'm having second thoughts.

The phrase may be cheesy, but its appropriate:
"Better safe than sorry!"

Originally Posted by MartinB

jumping speedo driven by a cable can be also caused by a frayed cable, mine used to do that right before the cable snapped...

Itís all down to what you think is right.
If you are ever stopped by the boys in blue, they cannot check the speedo as the vehicle is stationary and can only check a stationary car by law.
The only time a Speedo would be checked is if it was involved in an incident where the vehicle was tested on the move ( very rare).
Itís down to the definition of what a
Speedometer is, and that can be electrical or mechanical.

an instrument on an automobile or other vehicle for indicating the rate of travel in miles or kilometers per hour.

In the future I am sure they will all be triggered by GPS.
What people are looking at is an instrument, so the choice is what YOU feel comfortable with .

Hmm. I'm not so sure, given the recent issues with widespread jamming/spoofing of GPS systems in both Russia and the US. The rise in the use of GPS enabled drones for various criminal activities has led to so-called "geo-fencing" to prevent drones for entering certain airspace. It's not a well-defined boundary, so many GPS users around the Kremlin for example (including Uber, who use your smartphone GPS to know where the pick-up is) are being re-directed to a location some 25 miles away.

If all cars were equipped with GPS speedometers they would almost certainly also have a "black box" recorder for monitoring purposes. Say you strayed near a geo-fenced zone and were instantly teleported onto the M6 toll and were automatically charged? How would you prove that you weren't there?

I see a GPS speedo as being a driver aid only, like GPS, nothing more.